<PAGE>

                                                                 Exhibit 10.2

                                PROMISSORY NOTE
                                       

$500,000.00                                               Boston, Massachusetts
                                                              September 5, 1997

    FOR VALUE RECEIVED, the undersigned BioTransplant Incorporated, a Delaware
corporation (the "Borrower"), hereby promises to pay to the order of FLEET
NATIONAL BANK (the "Bank") the principal amount of Five Hundred Thousand and
00/100 ($500,000.00) Dollars or such portion thereof as may be advanced by the
Bank pursuant to Section 1.2 of that certain letter agreement of even date
herewith between the Bank and the Borrower (the "Letter Agreement") and remains
outstanding from time to time hereunder ("Principal"), with interest, at the
rate hereinafter set forth, on the daily balance of all unpaid Principal, from
the date hereof until payment in full of all Principal and interest hereunder.

    Interest on all unpaid Principal shall be due and payable monthly in
arrears, on the first day of each month, commencing on the first such date after
the advance of any Principal and continuing on the first day of each month
thereafter and on the date of payment of this note in full, at a fluctuating
rate per annum (computed on the basis of a year of three hundred sixty (360)
days for the actual number of days elapsed) which shall at all times be equal to
the Prime Rate, as in effect from time to time (but in no event in excess of the
maximum rate permitted by then applicable law), with a change in the aforesaid
rate of interest to become effective on the same day on which any change in the
Prime Rate is effective; provided, however, that (A) if a Eurodollar Interest
Rate (as defined in the Letter Agreement) shall have become applicable to all or
any portion of the outstanding Principal for any Interest Period (as defined in
the Letter Agreement), then interest on such Principal or portion thereof shall
accrue at said applicable Eurodollar Interest Rate for such Interest Period and
shall be payable on the Interest Payment Date (as defined in the Letter
Agreement) applicable to such Interest Period, and (B) if a COF Interest Rate
(as defined in the Letter Agreement) shall have become applicable to the
outstanding Principal, then interest on the outstanding Principal shall accrue
at said COF Interest Rate and shall be paid on the first day of each month. 
Overdue Principal and, to the extent permitted by law, overdue interest shall
bear interest at a fluctuating rate per annum which at all times shall be equal
to the sum of (i) two (2%) percent per annum plus (ii) the per annum rate
otherwise payable under this note with respect to the Principal which is overdue
(or as to which such interest is overdue) (but in no event in excess of the
maximum rate permitted by then applicable law), compounded monthly and payable
on demand.  As used herein, "Prime Rate" means that rate of interest per annum
announced by the Bank from time to time as its prime rate, it being understood
that such rate is merely a reference rate, not necessarily the lowest, which
serves as the basis upon which effective rates of interest are calculated for
obligations making reference thereto.  If the entire amount of any required
Principal and/or interest is not paid within ten (10) days after the same is
due, the Borrower shall pay to the Bank a late fee equal to five percent (5%) of
the required payment.

         All outstanding Principal shall be repaid by the Borrower to the Bank
in 36 equal consecutive monthly installments (each in an amount equal to 1/36th
of the total Principal outstanding at the close of business on March 31, 1998),
such installments to commence April 1, 

<PAGE>

1998 and to continue thereafter on the first Business Day (as defined in the 
Letter Agreement) of each month through and including March 1, 2001, on which 
date all then remaining Principal and all interest accrued but unpaid thereon 
will be due and payable in full.

    The Borrower may at any time and from time to time prepay all or any 
portion of any Term Loan (as defined in the Letter Agreement), but, as to 
Fixed Rate Loans (as defined in the Letter Agreement), only at the times and 
in the manner, and (under certain circumstances) with the additional 
payments, provided for in the Letter Agreement.  Any prepayment of Principal, 
in whole or in part, will be without premium or penalty (but, in the case of 
Fixed Rate Loans, may require payment of additional amounts, as provided for 
in the Letter Agreement). Each Principal prepayment shall be accompanied by 
payment of all interest on the prepaid amount accrued but unpaid to the date 
of payment.  Any partial prepayment of Principal will be applied against 
Principal installments in inverse order of normal maturity.

    Payments of both Principal and interest shall be made, in immediately
available funds, at the office of the Bank located at 75 State Street, Boston,
Massachusetts 02109, or at such other address as the Bank may from time to time
designate.

    The undersigned Borrower irrevocably authorizes the Bank to make or cause
to be made, on a schedule attached to this note or on the books of the Bank, at
or following the time of making any Term Loan and of receiving any payment of
Principal, an appropriate notation reflecting such transaction (including date,
amount and maturity) and the then aggregate unpaid balance of Principal. 
Failure of the Bank to make any such notation shall not, however, affect any
obligation of the Borrower hereunder or under the Letter Agreement.  The unpaid
Principal amount of this note, as recorded by the Bank from time to time on such
schedule or on such books, shall constitute presumptive evidence of the
aggregate unpaid principal amount of the Term Loans.

    The Borrower hereby (a) waives notice of and consents to any and all
advances, settlements, compromises, favors and indulgences (including, without
limitation, any extension or postponement of the time for payment), any and all
receipts, substitutions, additions, exchanges and releases of collateral, and
any and all additions, substitutions and releases of any person primarily or
secondarily liable, (b) waives presentment, demand, notice, protest and all
other demands and notices generally in connection with the delivery, acceptance,
performance, default or enforcement of or under this note, and (c) agrees to
pay, to the extent permitted by law, all reasonable costs and expenses,
including, without limitation, reasonable attorneys' fees, incurred or paid by
the Bank in enforcing this note and any collateral or security therefor, all
whether or not litigation is commenced.

    This note is the Term Note referred to in the Letter Agreement and is
entitled to the benefits of the Security Agreement (as defined in the Letter
Agreement).  This note is subject to prepayment as set forth in the Letter
Agreement (which, in the case of Fixed Rate Loans, may require the making of
certain additional payments, as provided for in the Letter Agreement).  The
maturity of this note may be accelerated upon the occurrence of an Event of
Default, as provided in the Letter Agreement.

                                       2
<PAGE>

    Executed, as an instrument under seal, as of the day and year first above
written.


CORPORATE SEAL                 BIOTRANSPLANT INCORPORATED


ATTEST:  
/s/Steven D. Singer            By: /s/Richard V. Capasso
-------------------------          ----------------------------
Secretary                          Name: Richard V. Capasso
                                   Title: Vice President, Finance and Treasurer

                                       3
<PAGE>
                         SECURITY AGREEMENT (EQUIPMENT)
                                            
    
    SECURITY AGREEMENT (EQUIPMENT) dated as of September 5, 1997 by and between
BioTransplant Incorporated, a Delaware corporation (the "Debtor") and Fleet
National Bank (the "Secured Party").
    
    WHEREAS, the Debtor and the Secured Party are parties to that certain
letter agreement of even date herewith (the "Letter Agreement") pursuant to
which the Secured Party may from time to time advance to the Debtor certain term
loans (the "Term Loans") for the purpose of acquisition of certain items of
equipment; and
    
    WHEREAS, the Term Loans are evidenced by the Debtor's $500,000 face
principal amount promissory note of even date herewith (the "Term Note") payable
to the order of the Secured Party; and
    
    WHEREAS, as a condition to the making of any Term Loan, the Secured Party
requires that the Debtor grant to the Secured Party a security interest in the
Collateral (as defined in Section 1);
    
    NOW, THEREFORE, for good and valuable consideration, the receipt and
sufficiency of which are hereby acknowledged, the parties hereby act and agree
as follows:
    
    1.   Definitions.  As used in this Security Agreement, the following terms
have the following meanings:
    
    "Collateral"  - All of the Equipment and all of the following:
    
    (a)  all warranties, licenses, contract rights and other rights and
    interests (other than any such warranties, licenses, contract rights or
    other rights or interests which are by their terms non-assignable)
    pertaining to any of the Equipment and/or the use thereof, all whether now
    or hereafter existing or owned by the Debtor or in which the Debtor shall
    now or hereafter have any interest; and
    
    (b)  all liens, guaranties, securities, rights, remedies and privileges
    pertaining to, and all proceeds (including, without limitation, insurance
    proceeds) of and all accessions to, any of the foregoing items of
    Collateral, all whether now or hereafter existing or owned by the Debtor or
    in which the Debtor shall now or hereafter have any interest.
    
    "Equipment"  - All of the items listed on Exhibit A hereto, as same may be
from time to time supplemented, and all accessions, additions, substitutions or
replacements to or for any of such items and all attachments, components,
accessories, parts and supplies relating thereto; all whether affixed or
moveable and wherever located, and whether now existing and owned by the Debtor
or herewith arising or acquired.

<PAGE>
    
    "Event of Default" - The occurrence of any one or more of the following:
(i) any "Event of Default" as defined in any Loan Document; or (ii) any
representation or warranty by the Debtor contained in this Security Agreement
shall prove to have been inaccurate or incomplete in any material respect on the
date when made; or (iii) the failure or default by the Debtor under any of
Subsections 4(a), 4(d), 4(e) or 4(g); or (iv) any failure by the Debtor to
perform or observe any of its other obligations or agreements under this
Security Agreement, which failure remains uncured for thirty (30) days after
notice thereof has been given to the Debtor.
    
     "Lien" - Any lien, charge, encumbrance or security interest, whether
voluntary or involuntary.
    
    "Loan Documents" - This Security Agreement, the Letter Agreement, the Term
Note and any other instruments, documents or other agreements made by the Debtor
with or in favor of the Secured Party in connection with any Term Loan, all
whether now existing or hereafter entered into or delivered.
    
    "Obligations" - Each Term Loan, the Term Note and any and all other
indebtedness, liabilities or obligations of the Debtor, joint or several, direct
or indirect, absolute or contingent, due or to become due, now existing or
hereafter arising, to or for the benefit of the Secured Party, which arise out
of or relate to any Term Loan.
    
    "Person"  - As defined in the Letter Agreement.
    
    "Premises" - All locations (whether owned, leased, operated or otherwise
used by the Debtor) in which any of the Equipment is or will be located, all of
which are listed on Exhibit B hereto together with the record owner of each such
location.
    
    "Security Agreement" - This Security Agreement (Equipment), as same may be
from time to time amended.
    
    "UCC" - The Uniform Commercial Code as in effect from time to time in
Massachusetts, except that with respect to Collateral located or deemed located
in any other jurisdiction, such term shall refer to the Uniform Commercial Code
as in effect in each such other jurisdiction.
    
    Any defined term used in the plural preceded by the definite article shall
be taken to encompass all members of the relevant class.  Any defined term used
in the singular preceded by "any" shall be taken to indicate any number of the
members of the relevant class.
    
    2.   Grant of Security Interest.  As security for the full and timely
satisfaction of the Obligations, the Debtor hereby grants to the Secured Party a
continuing security interest in the Collateral, and in each item thereof, all to
the maximum extent that the Debtor has an interest therein or at any time in the
future obtains such an interest.

                                       2
<PAGE>
    
    3.   Representations and Warranties.  The Debtor represents and warrants to
the Secured Party that:
    
    (a)  The execution, delivery and performance by the Debtor of this Security
Agreement, including the security interests herein granted or intended to be
granted, has been duly authorized by all necessary corporate and other action
and does not and will not:
    
       (i)  require any waiver, consent or approval of its shareholders, any
    governmental authority or any other Person;
       
       (ii)  contravene its charter or by-laws;
       
       (iii)  violate any provision of, or require any filing (other than the
    filing of financing statements under the UCC with respect to the security
    interests herein granted), registration, consent or approval under, any
    law, rule, regulation (including, without limitation, Regulation U), order,
    writ, judgment, injunction, decree, determination or award presently in
    effect having applicability to the Debtor;
       
       (iv)  result in a breach of or constitute a default or require any
    waiver or consent under any indenture or loan or credit agreement or any
    other material agreement, lease or instrument to which the Debtor is party
    or by which it or any of its properties may be bound; or
       
       (v)  result in, or require, the creation or imposition of any Lien
    (other than as created hereunder) upon or with respect to any of the
    properties now owned or hereafter acquired by the Debtor.
       
    (b)  This Security Agreement has been duly executed and delivered on behalf
of the Debtor and is a legal, valid and binding obligation of the Debtor.
    
    (c)  No Obligation has been or will hereafter be incurred on account of
personal, family or household purposes.
    
    (d)  The principal place of business and chief executive offices of the
Debtor are located at Third Avenue, Building 75, Charlestown Navy Yard,
Charlestown, Massachusetts 02129 and all of the books and records of the Debtor
are kept at that location.  Except as described on Exhibit B hereto, none of the
Collateral is kept at any other location.  The Debtor is a tenant in its
Premises and the record owner of each location constituting the Premises is as
set forth on Exhibit B hereto.
    
    (e)  The Debtor does not conduct business under any trade name or style
other than its corporate name.

                                       3
<PAGE>
    
    (f)  The Debtor owns the Collateral free and clear of all Liens except
(i) Liens in favor of the Secured Party, and (ii) the Lien of taxes not yet due
and payable or contested in good faith by the Debtor by appropriate proceedings
which serve to stay the enforcement thereof and as to which the Debtor has
established and is maintaining adequate reserves.
    
    (g)  This Agreement, coupled with the filing of appropriate UCC financing
statements with appropriate public offices, creates in favor of the Secured
Party a valid and perfected first priority security interest in all of the
Collateral.
    
    (h)  The Equipment has been accepted by the Debtor and is in good repair,
working order and condition, and the Debtor has not asserted (and the Debtor
knows of no basis for) any material warranty or other claim against any seller
or manufacturer thereof.
    
    4.   Covenants.  (a)  Payment and Performance.  The Debtor shall
unconditionally pay when due or within any applicable grace period (or on
demand, if so payable) each Obligation and shall duly and punctually perform
each Obligation.
    
    (b)  Further Assurances.  The Debtor will from time to time at its expense,
upon the Secured Party's request, promptly execute and deliver all such further
instruments and documents, and take all such further action, as the Secured
Party may reasonably request in order to perfect and/or protect the security
interests granted or intended to be granted hereby or to enable the Secured
Party to enforce its rights and remedies hereunder with respect to any
Collateral.  Without limitation of the foregoing, as a precondition to the
making of each Term Loan, the Debtor will execute and deliver to the Secured
Party (i) an agreement in form and substance satisfactory to the Secured Party
amending Exhibit A hereto by adding to same (without deleting any items of
Equipment previously shown thereon) a list of the additional items of equipment
to be purchased with the proceeds of such Term Loan and all such additional
items will thereupon be deemed included in the Equipment described in this
Security Agreement, (ii) UCC financing statements which will be sufficient (when
filed) to give the Secured Party a fully perfected first security interest in
all of such additional items, and (iii) a certificate of the Debtor confirming
that the representation and warranties contained in Section 3 above remain true
and correct as of the date of such Term Loan and taking into account such
additional items of equipment.
    
    (c)  Information.  The Debtor shall maintain complete and accurate records
of all of its Collateral and its dealings with respect thereto in accordance
with generally accepted accounting principles applied on a consistent basis. 
Upon reasonable notice from time to time at reasonable times during normal
business hours (and at any time and without notice after the occurrence and
during the continuance of an Event of Default), the Debtor shall permit the
Secured Party and its employees, representatives and agents access to the
Premises and the Secured Party shall have the right to inspect the Collateral
and make copies of such books and records. The Debtor shall from time to time
furnish to the Secured Party such information concerning the Collateral as the
Secured Party may reasonably request, and will promptly notify the Secured Party
if any 

                                       4
<PAGE>

representation or warranty of the Debtor in Section 3 hereof becomes 
inaccurate, incomplete or misleading in any material respect.
    
    (d)  Insurance.  The Debtor shall at its expense maintain fire and extended
coverage insurance policies insuring the Equipment, with responsible and
reputable insurance companies or associations, in amounts sufficient to provide
for full replacement cost coverage (with agreed amount endorsement), and in any
event not less than the amount necessary to avoid co-insurance. All such
insurance shall name the Secured Party as secured party and first loss payee. 
All policies of such insurance shall contain a provision forbidding cancellation
of such insurance either by the carrier or by the insured without at least 15
days' prior written notice to the Secured Party.  The Debtor shall upon the
Secured Party's request deliver to the Secured Party duplicate policies of such
insurance and/or binders, certificates or other evidence thereof (with evidence
of premiums having been paid) from the insurer or a reputable insurance broker. 
In case of any casualty, loss or damage to which the following sentence is not
applicable, the Debtor shall, in its sole discretion, either (a) make the
necessary repairs or replacements and shall be entitled to be reimbursed
therefor from and to the extent of the proceeds of such insurance or (b) have
all such insurance proceeds paid to the Secured Party for application in the
order provided in Subsection 8(c) below (provided that the option set forth in
this clause (b) cannot be elected unless such insurance proceeds are sufficient,
or the Debtor makes other payments, so that the Term Loan (or portion thereof)
which relates to the damaged or destroyed items of Equipment is paid in full). 
Upon the occurrence and during the continuance of any Event of Default, all
insurance payments in respect of Equipment shall be paid and applied as
specified in Subsection 8(c) below.
    
    (e)  Title; Sale or Removal of Collateral.  The Debtor shall not create or
suffer to exist any Lien in or on any of the Collateral, except (i) the Lien of
the Secured Party and (ii) the Lien of taxes not yet due and payable or
contested in good faith by the Debtor by appropriate proceedings which serve to
stay the enforcement thereof and as to which the Debtor has established and is
maintaining adequate reserves.  The Debtor shall not, without the Secured
Party's prior written approval, sell, transfer or remove any item of Collateral
from the Premises or otherwise dispose of any of the Collateral; provided that
the Debtor may move the Collateral to any other location if the Debtor gives the
Secured Party not less than thirty (30) days' prior written notice of such move
and provides all such financing statements, landlord's waivers and other
documentation as may be necessary to protect, perfect and/or confirm the first
priority security interests granted or intended to be granted in this Security
Agreement.  The Debtor (i) shall maintain books and records relating to
Collateral only as described in Subsection 3(d) above, (ii) will not move its
chief executive office or principal place of business from the location
described in Subsection 3(d) above, (iii) will not change its name or identity
(or use any trade name or style except as described in Subsection 3(e) above),
and (iv) will not make or suffer to be made any change in its corporate
structure until, in each case, after receipt of a certificate from the Secured
Party, signed by an officer thereof, stating that the Secured Party has, to its
satisfaction, obtained all documentation that it has reasonably requested in
order to obtain, maintain, perfect and/or confirm the first priority security
interests granted or intended to be granted herein.

                                       5
<PAGE>
    
    (f)  Maintenance and Use of Equipment.  The Debtor will maintain all
Equipment in good order and condition, making all necessary repairs thereto. 
The Debtor will not suffer any waste or destruction of any Equipment, nor use
any Equipment in violation of any applicable law or any insurance thereon.  The
Debtor will promptly restore or replace any Equipment damaged or destroyed by
fire or other casualty, or, at its option, may have all insurance proceeds
relating to such damaged or destroyed items of Equipment  paid to the Secured
Party for application in the order provided in Subsection 8(c) below (provided
that such option not to restore and replace Equipment cannot be elected unless
such insurance proceeds are sufficient, or the Debtor makes other payments, so
that the Term Loan (or portion thereof) which relates to the damaged or
destroyed items of Equipment is paid in full).  The Debtor shall promptly
furnish to the Secured Party a statement as to any casualty, loss or damage in
excess of $10,000 to any Equipment.
    
    (g)  Taxes.  The Debtor promptly shall pay, as they become due and payable,
all taxes, unemployment contributions and all other charges of any kind or
nature levied, assessed or claimed against the Debtor or the Collateral by any
Person whose claim could result in a Lien upon any of the Collateral, except to
the extent such taxes, contributions or other charges are being contested in
good faith and by appropriate proceedings which operate as a matter of law to
stay the enforcement of any such Lien and adequate reserves have been
established and are maintained by the Debtor.
    
    5.   Secured Party Appointed Attorney-in-Fact.  (a)  The Debtor hereby
irrevocably appoints the Secured Party as the Debtor's attorney-in-fact, but
effective only after the occurrence and during the continuance of any Event of
Default, with full authority in the name, place and stead of the Debtor, from
time to time in the Secured Party's discretion, to take any action and to
execute any instrument which the Secured Party may deem necessary or advisable
to accomplish the purposes of this Security Agreement, including, without
limitation, to obtain and adjust any insurance required pursuant to this
Security Agreement and/or the Letter Agreement.
    
    (b)  The power of attorney granted pursuant to this Section 5 is a power
coupled with an interest and shall be irrevocable until the Obligations are paid
indefeasibly in full.
    
    6.   Secured Party May Perform.  If the Debtor fails to perform any
agreement contained herein, the Secured Party may itself perform, or cause
performance of, such agreement, and the expenses of the Secured Party incurred
in connection therewith shall be payable by the Debtor as provided under Section
9 hereof, with interest as provided in the Letter Agreement.
    
    7.   Secured Party's Duties.  The powers conferred on the Secured Party
hereunder are solely to protect its interests in the Collateral and shall not
impose any duty upon it to exercise any such powers.  Except for the safe
custody of any Collateral actually in its possession and the accounting for
monies actually received by it hereunder, the Secured Party shall have no duty
as to any Collateral.  The Secured Party shall not be liable for any acts,
omissions, errors of judgment or mistakes of fact or law including, without
limitation, acts, omissions, errors or mistakes with respect to the Collateral,
except for those arising out of or in connection with the 

                                       6
<PAGE>

Secured Party's negligence, gross negligence or willful misconduct.  The 
Secured Party shall be deemed to have exercised reasonable care in the 
custody and preservation of the Collateral in its possession if the 
Collateral is accorded treatment substantially equal to that which the 
Secured Party accords its own like property, it being understood that the 
Secured Party shall be under no obligation to take any necessary steps to 
collect any Collateral or preserve rights against prior parties or any other 
rights pertaining to any Collateral, but may do so at its option, and all 
reasonable expenses incurred in connection therewith shall be for the sole 
account of the Debtor and shall be added to the Obligations.
    
    8.   Remedies.  If any Event of Default shall have occurred and be
continuing:
    
    (a)  The Secured Party may exercise in respect of the Collateral, in
addition to other rights and remedies provided for herein or otherwise available
to it, all the rights and remedies of a secured party under the UCC and also may
without limitation:
         
         (i)  require the Debtor to, and the Debtor hereby agrees that it will,
    at its expense and the upon reasonable request of the Secured Party,
    forthwith, assemble all or any part of the Collateral as directed by the
    Secured Party and make it available to the Secured Party at a place or
    places to be designated by the Secured Party which is or are reasonably
    convenient to the respective parties;
         
         (ii)  itself or through agents, without notice to any Person and
    without judicial process of any kind, enter the Debtor's Premises (or any
    other premises or location where any Collateral may be) and take physical
    possession of any Collateral or disassemble, render unusable and/or
    repossess any of the same, and the Debtor shall peacefully and quietly
    yield up and surrender the same; and
         
         (iii)  without notice except as specified below, sell, lease, assign,
    grant an option or options to purchase or otherwise dispose of the
    Collateral or any part thereof in one or more parcels at public or private
    sale, at any exchange, broker's board or at the Secured Party's offices or
    elsewhere, for cash, on credit or for future delivery, and upon such other
    terms as are commercially reasonable.
         
    (b)  The Secured Party may maintain possession of Collateral at the
Premises or remove the same or any part thereof to such places as the Secured
Party may elect.  The Debtor agrees that, to the extent notice of sale shall be
required by law, 10 days' prior written notice to the Debtor shall constitute
reasonable notification.  Notice of any public or private sale shall be
sufficient if it describes the Collateral to be sold in general terms, stating
the items or amounts thereof and the location and nature thereof, and (as to a
notice of public sale only) is published at least once in any newspaper selected
by the Secured Party and of general circulation in the locale of such sale, not
less than 10 days prior to the sale.  The Secured Party shall not be obligated
to make any sale of Collateral regardless of notice of sale having been given
and may be the purchaser at any such sale, if public, to the extent permitted by
applicable law, free from any right of redemption.  The Debtor shall be fully
liable for any deficiency.  The Secured Party may 

                                       7
<PAGE>

adjourn any public or private sale from time to time by announcement at the 
time and place fixed therefor, and such sale may, without further notice, be 
made at the time and place to which it was so adjourned.
    
    (c)  Any cash or other proceeds received by the Secured Party in respect of
any sale of, collection from, or other realization upon all or any part of the
Collateral, shall be applied by the Secured Party in the following order of
priorities:
    
    First, to the payment of the reasonable costs and expenses of any sale or
other reasonable expenses (including, without limitation, reasonable legal fees
and expenses), liabilities and advances made or incurred by the Secured Party in
connection therewith or referred to in Section 9 or provided for by the Letter
Agreement;
    
    Next, to payment of interest on and principal of the Term Note and other
charges relating thereto (in such order as may be provided for in the Letter
Agreement or as otherwise determined by the Secured Party);
    
    Next, to the payment of any other Obligations; and
    
    Finally, after payment in full of all Obligations, to the payment to the
Debtor or its successors or assigns, or to whomsoever may be lawfully entitled
to receive the same or as a court of competent jurisdiction may direct, of any
surplus then remaining of such cash.
    
    9.   Expenses and Indemnification.  The Debtor agrees to reimburse the
Secured Party for and to indemnify and hold harmless the Secured Party from and
against any and all liability, loss, damage, and all costs or expenses
(including, without limitation, reasonable fees and disbursements of counsel,
experts and agents) imposed on, incurred by or asserted against the Secured
Party arising out of or in connection with: preparation of this Security
Agreement, the documents relating hereto, or amendments, modifications or
waivers hereof; taxes (excluding any corporate excise or income taxes payable by
the Secured Party by reason hereof or otherwise) and other governmental charges
in connection with this Security Agreement and the Collateral; exercise of the
Secured Party's rights with respect to this Security Agreement and the
Collateral; any enforcement, collection or other proceedings resulting therefrom
or any negotiations or other measures to preserve the Secured Party's rights
hereunder; the custody or preservation of, or the sale of or other realization
upon, any of the Collateral; any failure by the Debtor to perform or observe any
of the provisions of this Security Agreement; any investigative, administrative
or judicial proceeding (whether or not the Secured Party is designated a party
thereto) relating to or arising out of this Security Agreement; or any
bankruptcy, insolvency or other similar proceeding relating to the Debtor,
unless the Secured Party was at fault with respect to such liability, loss,
damage, cost or expense or acted in bad faith with respect thereto.  The
Debtor's obligations under the preceding sentence shall constitute Obligations
and shall survive the termination of this Security Agreement.

                                       8
<PAGE>
    
    10.  Termination.  This Security Agreement shall remain in full force and
effect so long as any Obligation remains outstanding.  Upon the satisfaction in
full of all of the monetary Obligations, the Secured Party shall, at the
Debtor's expense, promptly execute and deliver to the Debtor all instruments of
assignment or otherwise as may be necessary to establish full title of the
Debtor to any of the Collateral, subject to any prior sale or other disposition
thereof pursuant to Section 8.  Until then, this Security Agreement shall itself
constitute conclusive evidence of the validity, effectiveness and continuing
force hereof, and any Person may rely hereon.
    
    11.  Waiver; Rights Cumulative.  No failure to exercise and no delay in
exercising, on the part of the Secured Party, any right or remedy hereunder or
otherwise shall operate as a waiver thereof, nor shall any single or partial
exercise thereof preclude any other or further exercise thereof or the exercise
of any other right or remedy.  Waiver by the Secured Party of any right or
remedy on any one occasion shall not be construed as a bar to or waiver thereof
or of any other right or remedy on any future occasion.  The Secured Party's
rights and remedies hereunder and under the Loan Documents shall be cumulative,
may be exercised singly or concurrently and are not exclusive of any rights or
remedies provided by law.
    
    The provisions of this Security Agreement are not in derogation or
limitation of any obligations, liabilities or duties of the Debtor under any of
the other Loan Documents or any other agreement with or for the benefit of the
Secured Party.  No inconsistency in default provisions between this Security
Agreement and any of the other Loan Documents or any such other agreement will
be deemed to create any additional grace period or otherwise derogate from the
express terms of each such default provision.  No covenant, agreement or
obligation of the Debtor contained herein, nor any right or remedy of the
Secured Party contained herein, shall in any respect be limited by or be deemed
in limitation of any inconsistent or additional provisions contained in any of
the other Loan Documents or any such other agreement.
    
    12.  Severability.  In the event that any provision of this Security
Agreement or the application thereof to any Person, property or circumstance
shall be held to any extent to be invalid or unenforceable, the remainder of
this Security Agreement and the application of such provision to Persons,
properties and circumstances other than those as to which it has been held
invalid or unenforceable shall not be affected thereby, and each provision of
this Security Agreement shall be valid and enforceable to the fullest extent
permitted by law.
    
    13.  Binding Effect; Assignment.  This Security Agreement shall be binding
upon the Debtor and its successors and assigns and shall inure to the benefit of
the Debtor and the Secured Party and their respective successors and assigns.
    
    14.  Notices.  All notices and other communications under or relating to
this Security Agreement shall be given in the manner and to the addresses of the
parties provided for in Section 6.3 of the Letter Agreement.

                                       9
<PAGE>
    
    15.  Headings.  Section headings in this Security Agreement are included
herein for convenience of reference only and shall not constitute a part of this
Security Agreement for any other purpose.
    
    16.  Governing Law.  This Security Agreement shall be governed by, and
construed and enforced in accordance with, the laws of The Commonwealth of
Massachusetts, except that the creation, perfection and enforcement of security
interests in any Collateral located in jurisdictions other than Massachusetts
will be governed by the laws of the respective jurisdictions in which such
Collateral is located.
    
    IN WITNESS WHEREOF, the Debtor and the Secured Party have caused this
Security Agreement to be executed, as an instrument under seal, by their
respective officers thereunto duly authorized, as of the date first above
written.
    
    
                               BIOTRANSPLANT INCORPORATED
     
     
                               By /s/Richard V. Capasso
                                  ------------------------------------
                                  Name: Richard V. Capasso
                                  Title: Vice President, Finance and Treasurer
     
     
                               FLEET NATIONAL BANK
     
     
                               By /s/Kimberly Martone
                                  -----------------------------------
                                  Name: Kimberly Martone
                                  Title: Vice President

                                       10
<PAGE>

    
    EXHIBIT A - Equipment list
    
    
    EXHIBIT B - Locations in which Equipment is located, including owners of
real estate

<PAGE>

                                      EXHIBIT A
                                    EQUIPMENT LIST
                                           
                                           
                                           
                        [To be provided with each Term Loan.]
                                           

<PAGE>

                                  EXHIBIT B
                           COLLATERAL LOCATIONS
                                           
Location                                       Record Owner

Third Avenue, Building 75             The Boson Redevelopment
Charlestown Navy Yard                 Authority (ground lessee:  BioLease, Inc.)
Charlestown, MA  02129